1. Field of the Invention
This invention resides in the field of steel alloys, particularly those of high strength, toughness, corrosion resistance, and cold formability, and also in the technology of the processing of steel alloys to form microstructures that provide the steel with particular physical and chemical properties.
2. Description of the Prior Art
Steel alloys of high strength and toughness and cold formability whose microstructures are composites of martensite and austenite phases are disclosed in the following United States patents, each of which is incorporated herein by reference in its entirety:                4,170,497 (Gareth Thomas and Bangaru V. N. Rao), issued Oct. 9, 1979 on an application filed Aug. 24, 1977        4,170,499 (Gareth Thomas and Bangaru V. N. Rao), issued Oct. 9, 1979 on an application filed Sep. 14, 1978 as a continuation-in-part of the above application filed on Aug. 24, 1977        4,619,714 (Gareth Thomas, Jae-Hwan Ahn, and Nack-Joon Kim), issued Oct. 28, 1986 on an application filed Nov. 29, 1984, as a continuation-in-part of an application filed on Aug. 6, 1984        4,671,827 (Gareth Thomas, Nack J. Kim, and Ramamoorthy Ramesh), issued Jun. 9, 1987 on an application filed on Oct. 11, 1985        6,273,968 B1 (Gareth Thomas), issued Aug. 14, 2001 on an application filed on Mar. 28, 2000        
The microstructure plays a key role in establishing the properties of a particular steel alloy, and thus strength and toughness of the alloy depend not only on the selection and amounts of the alloying elements, but also on the crystalline phases present and their arrangement. Alloys intended for use in certain environments require higher strength and toughness, and in general a combination of properties that are often in conflict, since certain alloying elements that contribute to one property may detract from another.
The alloys disclosed in the patents listed above are carbon steel alloys that have microstructures consisting of laths of martensite alternating with thin films of austenite. In some cases, the martensite is dispersed with fine grains of carbides produced by autotempering. The arrangement in which laths of one phase are separated by thin films of the other is referred to as a “dislocated lath” structure, and is formed by first heating the alloy into the austenite range, then cooling the alloy below the martensite start temperature Ms, which is the temperature at which the martensite phase first begins to form, into a temperature range in which austenite transforms into packets consisting of martensite laths separated by thin films of untransformed, stabilized austenite. This is accompanied by standard metallurgical processing, such as casting, heat treatment, rolling, and forging, to achieve the desired shape of the product and to refine the alternating lath and thin film arrangement. This microstructure is preferable to the alternative of a twinned martensite structure, since the lath structure has greater toughness. The patents also disclose that excess carbon in the lath regions precipitates during the cooling process to form cementite (iron carbide, Fe3C) by a phenomenon known as “autotempering.” The '968 patent discloses that autotempering can be avoided by limiting the choice of the alloying elements such that the martensite start temperature Ms is 350° C. or greater. In certain alloys the carbides produced by autotempering add to the toughness of the steel while in others the carbides limit the toughness.
The dislocated lath structure produces a high-strength steel that is both tough and ductile, qualities that are needed for resistance to crack propagation and for sufficient formability to permit the successful fabrication of engineering components from the steel. Controlling the martensite phase to achieve a dislocated lath structure rather than a twinned structure is one of the most effective means of achieving the necessary levels of strength and toughness, while the thin films of retained austenite contribute the qualities of ductility and formability. Obtaining such a dislocated lath microstructure rather than the less desirable twinned structure is achieved by a careful selection of the alloy composition, which in turn affects the value of Ms.
The stability of the austenite in the dislocated lath microstructure is a factor in the ability of the alloy to retain its toughness, particularly when the alloy is exposed to harsh mechanical and environmental conditions. In certain conditions, austenite is unstable at temperatures above about 300° C., tending to transform to carbide precipitates which render the alloy relatively brittle and less capable of withstanding mechanical stresses. This instability is one of the issues addressed by the present invention.